1. Field of the Invention
The present invention relates to the fabrication field of semiconductor devices, and more particularly to a method for forming a gate electrode of a compound semiconductor device using lift-off an insulating film.
2. Description of the Related Art
FIGS. 1a to 1d are cross-sectional views showing a method of fabricating a field effect type compound semiconductor devices, such as high electron mobility transistor (HEMT) or metal semiconductor field effect transistor (MESFET), according to a conventional method.
First, as shown in FIG. 1a, a GaAs buffer layer 2, a AlGaAs/GaAs superlattice buffer layer 3, a channel layer 4, a spacer layer 5, a semiconductor layer 6, and a n type GaAs ohmic contact layer 7 are successively grown on the semi-insulating GaAs substrate 1.
Next, as shown in FIG. 1b, a resist consisting of deposited polymethylmethacrylate (PMMA) and co-polymer is deposited by spin coating on the GaAs ohmic layer 7. An electron beam irradiates the resist, which is developed to form a resist pattern having a T-type profile. The GaAs ohmic layer 7 is removed by dry etching using the resist pattern as a mask.
Next, as shown in FIG. 1c, Ti/Pt/Au metal film 9 is deposited on the resist pattern 8 and semiconductor layer 6. T-type gate electrode 10 is formed within the resist pattern 8 having a T-type profile.
Finally, as shown in FIG. 1d, as the metal film deposited the inside and top of the resist pattern is simultaneously removed by lift-off method, T-type gate electrode 10 and GaAs ohmic layer 7 are exposed. sequentially, Ohmic metal (AuGe/Ni/Au) electrode 11 of source and drain are produced by evaporation, self-aligning, using the T-type gate electrode 10 as a mask. AuGe/Ni/Au films are formed to a thickness of 1000.ANG. to 2000 .ANG., 400.ANG. to 1000.ANG., and 400.ANG. to 1000.ANG., respectively. Sequentially, the field effect type compound semiconductor devices, such as HEMT, MESFET is accomplished by rapid annealing at the temperature of 430.degree. C. for 20 sec.
As described above, the conventional semiconductor device is formed a resist pattern having a T-type profile using PMMA and co-polymer, sequentially deposited a metal film for gate electrode. In the case of deposition of the refractory metal on the resist pattern having a T-type profile in order to form the gate electrode, the refractory metal gate electrode is difficult to form stably due to melting of the resist. Also, the T-type gate electrode of materials such as Ti/Pt/Au shows an unstable device characteristics due to its deterioration, thereby annealing at the high temperature, after forming by self-aligning method an ohmic metal electrode.
In the formation process by self-aligning method a source and drain using the T-type gate electrode as a mask, the device reliability decreases due to the interconnection between gate and ohmic metal electrodes, because the insulating film is not formed at a lower part of gate electrode.